1. Field of the Invention and Related Art Statement
The present invention relates to a glass melting furnace and, more particularly, to a vertical electric furnace which is capable of producing high-quality glass with efficiency and stability.
A conventional vertical electric glass melting furnace generally has a structure in which a material entrance 1 is provided at the upper most portion of the furnace, a glass material (batch) 3 supplied to the surface of a molten glass 4 in the furnace is melted by heating the molten glass 4 by electricity directly passing through the glass by means of electrodes 5 which are immersed therein, and the molten glass after it is refined is conveyed to a working section from a glass exit 2 provided at the bottom portion of the furnace, as shown in FIGS. 4 to 6. The rod electrodes 5 may be horizontally inserted from the side wall of the furnace, as shown in FIG. 4, vertically inserted from the bottom portion of the furnace, as shown in FIG. 5, or inserted from the upper portion of the furnace through the layer of the glass material 3, as shown in FIG. 6.
In such an electric furnace, the surface of the molten glass 4 is covered with the layer of the glass material (batch) 3 so as to form what is called a "cold top" for the purpose of efficient utilization of thermal energy. The glass melts and is refined as it approaches the bottom of the furnace.
In order to obtain high-quality glass by such an electric furnace, it is necessary that the glass supplied to the working section through the exit 2 at the bottom portion of the furnace has a sufficient thermal history and does not contain any bubble or unmelted substance. The thermal history of the glass reaching the exit 2 is determined by the temperature of the glass in the furnace and the flow of the molten glass. Since the flow of the molten glass is mainly subject to the convection current caused by the temperature distribution of the glass in the furnace, the most important thing in the operation of the furnace for producing high-quality glass is to maintain the furnace in a stable thermal equilibrium.
It is difficult, however, to make the distribution of electric current uniform in the plane of the furnace in a conventional vertical electric furnace which utilizes the Joule heat produced on the glass by the rod electrodes for heating glass. Particularly, since the current density becomes high in the vicinity of the tip portions of the electrodes, the temperature of the glass between the tips of the electrodes is apt to be raised. Since the electric resistance of glass is reduced as the temperature thereof becomes higher, more and more current flows to the high-temperature portion, thereby disadvantageously furthering the local heating. Such a non-uniform temperature distribution causes convection current of the molten glass in the furnace and, as a result, the molten glass having a short thermal history, namely, the molten glass containing bubbles and unmelted substances due to the insufficient heating flows out of the exit, which is supplied to the working section and may cause a defect in the product.
In addition, the non-uniform temperature distribution and the convection current produced thereby on the molten glass lead to local melting of the batch on the surface of the molten glass, so that there produces some portions at which the molten glass is not covered with the batch layer, thereby increasing the heat loss and lowering the glass temperature. In a furnace for melting glass by heating it with Joule heat, the temperature control of the molten glass is very difficult for the above-described reasons. Thus, it is almost impossible to prevent the non-uniform temperature distribution of the molten glass.
These problems are serious in the case of melting glass which does not contain any alkaline component and, hence, has a large electric resistance. That is, since such a non-alkaline glass has a large electric resistance, it is necessary to reduce the distance between the electrodes in order to provide the power necessary for melting the glass, so that the non-uniform temperature distribution becomes more prominent.